1. Field of the Invention
The present invention relates to an apparatus for converting a low-resolution signal applied from a host into a video signal having different-resolution, and a display device having the same.
2. Background Art
Display devices, such as a liquid crystal display (LCD) device and plasma display device, have a plurality of pixels for displaying an image, wherein the pixel brightness is controlled in accordance with video information provided from a host.
An exemplary active matrix LCD device, which is provided with an LCD control unit and an LCD panel, displays an image on the screen of the LCD panel in a such manner that pixels are turned on/off by means of switching elements corresponding respectively to the pixels. The LCD control unit converts analog color signals from a host (e.g., a personal computer) into digital RGB color signals and generates a horizontal output signal, a vertical output signal and a dot (i.e., pixel) clock signal in response to horizontal synchronization signals and vertical synchronization signals from the host. The LCD panel has an LCD driving unit therein. The digital RGB color signals, dot clock signal, horizontal output signals and vertical output signals, which are provided from the LCD control unit, are supplied to the LCD driving circuit incorporated in the LCD panel.
An exemplary LCD control unit, which is provided to control the LCD panel, has a phase locked loop (PLL) circuit and an analog-to-digital converter (ADC). When the PLL circuit receives a horizontal synchronization signal, it generates a horizontal output signal and a dot clock signal. Also the ADC circuit converts analog color signals of R (red), G (green) and B (blue) from the host into digital color signals of R, G and B, respectively, which are supplied to the LCD driving circuit. The horizontal output signal Hout is produced from the horizontal synchronization signal, and the frequency of the horizontal output signal is equal to that of the horizontal synchronization signal. Meanwhile, the polarity of the horizontal synchronization signal being fed to the PLL circuit may be changed in accordance with the kinds of the host, and the PLL circuit outputs the horizontal output signal having a predetermined polarity. For example, in the LCD device having the driving circuit which is operated in synchronization with the horizontal output signal having negative polarity, even though the horizontal synchronization signal of positive polarity from the host is supplied to the PLL circuit in the LCD device, the PLL circuit supplies the horizontal output signal of negative polarity for the LCD driving circuit. The PLL circuit, as well known in the art, has a phase detector, a voltage controlled oscillator (VCO), a divider, and an output generator.
In general, the exemplary LCD device embodies a single display mode, for example, Video Graphics Array (VGA) mode, Super VGA (SVGA) mode or extended Graphics Array (XGA) mode. Accordingly, if the VGA mode video signals of 640.times.480 active resolution are provided to the XGA mode supporting LCD device having the active resolution of 1024.times.768, an image is displayed on only a partial area of the LCD screen, and is not displayed on the screen's remaining area. If the SVGA mode signals having the active resolution of 800.times.600 are also provided to the XGA LCD device, the results are similar to the above case. Thus, one of several problems in the exemplary LCD device, if low-resolution display mode signals from the host are fed to an LCD device capable of supporting high-resolution display mode signals, is that an image is partially displayed on the LCD screen.